Unsaturated hydantoins are important precursors in the synthesis of some amino acids. These unsaturated hydantoins can be formed by any number of reactions with one of the more commonly used being a condensation reaction between an aldehyde and a substituted or unsubstituted hydantoin. In this reaction, an ethylenic bond is formed between the non-carbonyl, or C-5, carbon of the hydantoin moiety and the carbonyl carbon of the original aldehyde. Further reduction, or hydrogenation, of this ethylenic linkage is a necessary step in the synthesis of some amino acids. This step must be done without hydrogenation of any of the aromatic or aliphatic substituents of the hydantoin moiety other than at this ethylenic linkage. Previously, this hydrogenation step has been done using hydrogen and a nickel catalyst under high pressure or by using hydrogen and a very expensive noble metal catalyst such as palladium or platinum under little or no pressure.
The use of one or more of these techniques is reported in a number of U.S. patents. In U.S. Pat. No. 2,605,282, 5-vanillylidenehydantoin is reduced to the 5-vanillylhydantoin by dissolving the unsaturated hydantoin in an aqueous solution containing 4 to 10 percent by weight of sodium hydroxide (75 mole percent of the unsaturated hydantoin) and shaking the mixture with hydrogen under pressure in the presence of a palladium containing hydrogenation catalyst. The reduction is carried out at a temperature of 25.degree. to 40.degree. C. at a pressure of 60 pounds per square-inch gauge or higher for 1 to 4 hours.
In U.S. Pat. No. 2,479,065, 5-benzalhydantoin is reduced to 5-benzylhydantoin using a caustic activated nickel aluminum alloy catalyst, methanol as a solvent and pressures of from 750 to 760 atmospheres. One disadvantage of the above method is the use of extremely high pressures to complete the hydrogenation in a short reaction time. The above mentioned patent does not specifically define the type of nickel aluminum alloy to be caustic activated or the degree of caustic activation. Although nickel aluminum alloys are commonly employed catalysts in hydrogenation procedures, a distinction must be drawn between a nickel alloy catalyst and a particular class of nickel type catalyst called Raney nickel catalyst. The accepted method of making the latter catalyst involves reacting the nickelaluminum alloy with caustic to remove the aluminum and then washing the precipitated nickel with water to remove essentially all the caustic to produce a spongy nickel catalyst. [Ind. and Eng. Chem. 33 1199 (1940)]: Hereinafter, the term Raney nickel catalyst refers to the form of nickel catalyst produced by the above procedure.
Prior work on the process of low-pressure hydrogenation of unsaturated hydantoins as disclosed in the parent application, Ser. No. 641,886, was done at a 10 percent concentration in water. Engineering studies indicated a large savings in production costs could be achieved by increasing the concentration to 20 percent. However, at 20 percent concentration level, severe frothing and foaming occurs at the upper level of the liquid phase with the hydrogen gas. This pasty froth expands and fills up all the free space in the reaction vessel and even into accessory equipment. The frothing occurs regardless of the scale of the reaction. The reduction in agitation speed or the increase in reaction vessel and reaction size (from 250 milliliter to 22 liter flasks), or the use of baffles, or the use of surface active agents does not overcome the problem. The use of a cylindrical reaction vessel in place of the spherical laboratory reaction flask is of some assistance in overcoming the problem. The use of a stoichiometric amount of sodium hydroxide (100 mole percent) and increased reaction temperature has also been attempted. None of these changes eliminates the frothing problem.